hemant nanavati | Indian Institute of Technology Bombay (original) (raw)
Papers by hemant nanavati
arXiv (Cornell University), May 25, 2024
We identify for visco-elasto-plastic (VEP) glassy polymers, physical phenomena during Berkovich n... more We identify for visco-elasto-plastic (VEP) glassy polymers, physical phenomena during Berkovich nanoindentation, a locally imposed deformation. Live visuals via in situ nanoindentation indicate mainly sink-in during loading, with pileup after unloading. Scanning Probe Microscopy (SPM) indicates significant volume conserving upflow below the tip, for these high , compliant materials, with compliance correlated high geometric fractional contact, m~0
Bulletin of the American Physical Society, 2016
densely crosslinked networks, macroscopic yield is a transition from deformations of bond lengths... more densely crosslinked networks, macroscopic yield is a transition from deformations of bond lengths and angles, to cooperative deformation of multiple effective network chains via bond torsions. In this work, we examine this yield in terms of the "activation number", ν, of microscopic effective chains between crosslinks. ν is the number of effective network chains, in one Eyring activation volume, V*. It is thus a measure of the number of network chains activated at yield, for cooperative deformation. Microcompression experiments have been performed on SU-8 micropillars, to determine its V* value. SU-8 is an important epoxy thermoset, which is used extensively in the microelectronics industry, in microfluidics and microelectromechanical systems (MEMS). The effective chain length based on Arruda and Boyce's 8-chain model, compares well with the rms length, obtained by chain conformer analyses. We find that ν ∼ 2-4, at room temperature, for DGEBA-based epoxies including SU-8 and DGEBA-amine networks, over a range of network junction functionalities and V*. That ν corresponds very well with the reduced temperature, T/Tg, also demonstrates its viability as a molecular descriptor of yield in densely crosslinked thermosets.
Polymer, 2016
In densely crosslinked glassy thermosets, at yield, bond length and bond angle deformations, tran... more In densely crosslinked glassy thermosets, at yield, bond length and bond angle deformations, transition to cooperative deformation of multiple effective network chains via bond torsions. We introduce here the activation number, n, the ratio of the Eyring activation volume, V*, to the network chain volume, V c , as a measure of the number of network chains, 'activated' at yield for cooperative deformation. We find that n~2e4, at room temperature, for DGEBA-based epoxies including SU-8 and DGEBA-amine networks, over a range of network junction functionalities and V*. This first reported analysis of yield at various strain rates, of the octafunctional epoxy, SU-8, is via our compression experiments on fabricated micropillars. Molecular validation of our bulk density based analysis to determine V c , is achieved by mapping to the eight-chain network model of Arruda and Boyce. We thus identify the activation number as a universal molecular descriptor of yield in densely crosslinked glassy thermosets.
Macromolecules, 2006
We present dynamic Monte Carlo lattice simulations of the coil to globule collapse of single chai... more We present dynamic Monte Carlo lattice simulations of the coil to globule collapse of single chains of a copolymer comprising monomer units, m and c, wherein there is a net attractive interaction between c-units. As the copolymer is cooled, the solvent quality becomes poorer, and the size of the chain decreases, driven by the net mm and cc attractions. The strong cc attraction increases the overall solvophobicity of the chain relative to a homopolymer and, therefore, copolymers collapse more abruptly and at a higher effective temperature relative to homopolymers. We compare copolymers with homopolymers by rescaling collapse data to the same θ values to account for the effect of overall solvophobicity. This comparison shows that the behavior of copolymers and the corresponding homopolymers is identical as the chain size reduces from high temperatures to the θ value. Beyond θ, copolymers with c-content < ∼50% collapse more abruptly than either homopolymer, after accounting for the difference in overall solvophobicity. Collapse of copolymers containing higher c-content is dominated entirely by the cc attractions, and these chains behave qualitatively like homopolymers with a higher effective solvophobicity. Analysis of the chain structure during collapse provides a structural reason for the qualitative change in copolymer collapse at low c-content. When such copolymers are cooled below θ, the c-units rapidly aggregate to form an isotropic, compact core surrounded by an anisotropic solvated shell of m-units. The shell densifies as the copolymer is further cooled, but remains anisotropic for the finite chain sizes investigated.
The Journal of Chemical Physics, 2009
The effect of “sticky” additives (viz., those that have attractive interactions with the polymer)... more The effect of “sticky” additives (viz., those that have attractive interactions with the polymer) on polymer crystallization, has been investigated by dynamic Monte Carlo (DMC) simulations. Additive-polymer attractive interactions result in a slowing down of the polymer chain diffusivity in the melt state. Our results show that with increasing additive stickiness, polymer crystallinity decreases monotonically, and thinner crystallites form, viz., crystallization is inhibited by the presence of sticky additives. Unusually, the observed “specific heat” peak at the phase transition shows nonmonotonic behavior with additive stickiness, and exhibits a maximum for intermediate values of additive stickiness. While the origins of this unexpected behavior are not clear, we show that it correlates with a large interchange between crystalline and amorphous states of the monomers, in the vicinity of the additives. At this intermediate additive stickiness, we also find that crystallization follo...
The Journal of Chemical Physics, 2007
Monte Carlo simulations show that copolymers with uniformly (or periodically) distributed sticky ... more Monte Carlo simulations show that copolymers with uniformly (or periodically) distributed sticky comonomers collapse “cooperatively,” abruptly forming a compact intermediate comprising a monomer shell surrounding a core of the aggregated comonomers. In comparison, random copolymers collapse through a relatively less-compact intermediate comprising a comonomer core surrounded by a fluffy monomer shell that densifies over a wide temperature range. This difference between the collapse pathways for random and uniform copolymers persists to higher chain lengths, where uniform copolymers tend to form multiple comonomer cores. In this paper, we describe the formation of such an intermediate state, and the subsequent collapse, by recognizing that these arise from the expected balance between comonomer aggregation enthalpy and loop formation entropy dictated by the chain microstructure.
A number of people have contributed to this work and helped me see it through completion. I'm in ... more A number of people have contributed to this work and helped me see it through completion. I'm in their debt. I'm especially grateful to my advisor, Dr. Sue Ann Bidstrup who gave me the opportunity to pursue research in this area. Dr. Bidstrup also provided the guidance and support without which this thesis would not have been possible. She devoted extra time and energy and effort to ensure that in spite of myself, this thesis reflected my best effort. For this too I'm grateful. I would like to extend my sincere appreciation to the Dr. A. S. Abhiraman and Dr. Paul Kohl for serving on my Reading Committee. Their advice and suggestions were very helpful in developing this work. Sincere thanks to Dr. Abhiraman for the moral support he gave me and his concern. Materials donation from E. 1. Du Pont de Nemours and Company (Inc.), Electronics Division and financial support from Manufacturing Research Center at Georgia Tech and the Presidential Young Investigator Program of the National Science Foundation is gratefully acknowledged. Special thanks are extended to my group-mates and friends-Jim Companik, Linda Lin and Joycelyn Simpson. Your contributions from the brainstorming sessions, personal moral support and cooperation have been invaluable. I'm especially indebted to my officemate Linda Lin. She has always been there (being available at ungodly hours to prepare the samples for FTIR spectroscopy being one of the many cases in point). Her friendship has been a privilege. Special thanks also to Linda Lin and office-mate and friend Tom Hodge for assistance in proof reading this thesis. I would like to take this opporturlity to thank Steve Hardaker for training me to use the FTIR Spectrometer and helping me learn to interpret spectroscopy data. Life in the USA has been a wonderful experience. I'll always treasure the many friendships I've made in my two and a half year stay. Like I always tell the new Indian students before they find out for themselvesthe natives are a very friendly bunch. Numerous special friendships have developed within the Indian community also during the course of this phase of my pursuit of graduate study and have made my stay here quite enjoyable: Rangaswamy Rajamanickam (Ranga), Nilesh Shroff, Pankaj Upadhyay, Ram Kumar Ramesh (Ravan), Vishnu Vandana, Anuradha Khawas (Anu), Bodhisattwa Mukherjee (Bodhi), Arun Mangalam (Sacold friend and new too), the Dalal family (Satishbhai, Smitaben, Tanvi and Nija). I extend my sincere gratitude to my parents, Jayantkumar and Uma Nanavati, my brother, Kaushik Nanavati, my late uncle, Vasantlal Nanavati and my grandmothers, Navnitben Nanavati and Nirmala Khandwala for their continued love and moral support from across the seven seas. You are all priceless jewels in the treasury of my life.
Journal of Computational Chemistry, Sep 1, 2007
Database-assisted ab initio protein structure prediction methods have exhibited considerable prom... more Database-assisted ab initio protein structure prediction methods have exhibited considerable promise in the recent past, with several implementations being successful in community-wide experiments (CASP). We have employed combinatorial optimization techniques toward solving the protein structure prediction problem. A Monte Carlo minimization algorithm has been employed on a constrained search space to identify minimum energy configurations. The search space is constrained by using radius of gyration cutoffs, the loop backbone dihedral probability distributions, and various secondary structure packing conformations. Simulations have been carried out on several sequences and 1000 conformations have been initially generated. Of these, 50 best candidates have then been selected as probable conformations. The search for the optimum has been simplified by incorporating various geometrical constraints on secondary structural elements using distance restraint potential functions. The advantages of the reported methodology are its simplicity, and modifiability to include other geometric and probabilistic restraints.
In densely crosslinked glassy thermosets, at yield, bond length and bond angle deformations, tran... more In densely crosslinked glassy thermosets, at yield, bond length and bond angle deformations, transition
to cooperative deformation of multiple effective network chains via bond torsions. We introduce here the
activation number, n, the ratio of the Eyring activation volume, V*, to the network chain volume, Vc, as a
measure of the number of network chains, ‘activated’ at yield for cooperative deformation. We find that
n ~ 2-4, at room temperature, for DGEBA-based epoxies including SU-8 and DGEBA-amine networks,
over a range of network junction functionalities and V*. This first reported analysis of yield at various
strain rates, of the octafunctional epoxy, SU-8, is via our compression experiments on fabricated
micropillars. Molecular validation of our bulk density based analysis to determine Vc, is achieved by
mapping to the eight-chain network model of Arruda and Boyce. We thus identify the activation number
as a universal molecular descriptor of yield in densely crosslinked glassy thermosets.
We present dynamic Monte Carlo simulations of the collapse of copolymers containing sticky comono... more We present dynamic Monte Carlo simulations of the collapse of copolymers containing sticky comonomers, c. There is a qualitative difference in the transition depending on c content. For c content >˜ 50%, copolymer collapse is qualitatively similar to that observed for homopolymers, when rescaled to account for comonomer solvophobicity. However, collapse of copolymers with c < ˜50% is qualitatively steeper
The effect of additive particles on polymer crystallization has been investigated using lattice d... more The effect of additive particles on polymer crystallization has been investigated using lattice dynamic Monte Carlo simulation. Additives are compatible with the polymer matrix (viz. there is an attractive ``sticky'' interaction between additives and monomers) and, additive particles have the same size as a monomer. Polymer crystallization is strongly influenced by both additive fraction, x and the additive-monomer interaction strength,
Polyhedron, 2007
Phenoxy-imines are a versatile class of ligands that display a truly impressive range of diverse ... more Phenoxy-imines are a versatile class of ligands that display a truly impressive range of diverse applications spanning from bioinorganic chemistry [1] to coordination chemistry [2], chemical catalysis [3], [4], [5], [6] and [7], and materials related applications [8] and [9]. Equally ...
Polymer Composites, 2011
Solid state polymerization (SSP) is an established procedure to increase the molecular weight (MW... more Solid state polymerization (SSP) is an established procedure to increase the molecular weight (MW) of polyesters [1] (eg, poly (ethylene terephthalate) (PET) [2], poly (butylene terphthalate) [3]), polycarbonates [4], polyamides [5] such as nylon [6, 7], etc. During SSP, ...
The freely orienting model of a polymer chain is generalized by considering the distribution of v... more The freely orienting model of a polymer chain is generalized by considering the distribution of vector lengths and polarizabilities of the statistical segments in the chain. These distributions for a statistical segment have been computed for polyethylene and poly(cis-isoprene) by using Flory's RIS (Rotational Isomeric States) approximation and Denbigh's bond polarizabilities. Two methods, based on distributions of finite chains of freely orienting constant length segments, are used to compute the chain length distribution from the length distribution of statistical segments, (i) an exact method, adopting Chandrasekhar's approach of Fourier transforms of segment vector distributions, and (ii) an alternative approach, based on the generalization of James' and Guth's expression for work of chain deformation. Using the computationally more viable latter approximation, the moments of the chain vector length distributions and chain polarizability are determined f...
Polymer Engineering & Science, 2011
Polymer Engineering & Science, 2011
Polymer Chemistry, 2010
We report here our investigations of the mechanism and kinetics of N-heterocyclic molecules as in... more We report here our investigations of the mechanism and kinetics of N-heterocyclic molecules as initiators for PLA synthesis. This is a step towards employing life saving drugs based on N-heterocyclic functional groups, to generate carrier-drug complexes in situ, via ring ...
Macromolecules, 2006
We present dynamic Monte Carlo lattice simulations of the coil to globule collapse of single chai... more We present dynamic Monte Carlo lattice simulations of the coil to globule collapse of single chains of a copolymer comprising monomer units, m and c, wherein there is a net attractive interaction between c-units. As the copolymer is cooled, the solvent quality becomes poorer, and the size of the chain decreases, driven by the net m-m and c-c attractions. The strong c-c attraction increases the overall solvophobicity of the chain relative to a homopolymer and, therefore, copolymers collapse more abruptly and at a higher effective temperature relative to homopolymers. We compare copolymers with homopolymers by rescaling collapse data to the same θ values to account for the effect of overall solvophobicity. This comparison shows that the behavior of copolymers and the corresponding homopolymers is identical as the chain size reduces from high temperatures to the θ value. Beyond θ, copolymers with c-content < ∼50% collapse more abruptly than either homopolymer, after accounting for the difference in overall solvophobicity. Collapse of copolymers containing higher c-content is dominated entirely by the c-c attractions, and these chains behave qualitatively like homopolymers with a higher effective solvophobicity. Analysis of the chain structure during collapse provides a structural reason for the qualitative change in copolymer collapse at low c-content. When such copolymers are cooled below θ, the c-units rapidly aggregate to form an isotropic, compact core surrounded by an anisotropic solvated shell of m-units. The shell densifies as the copolymer is further cooled, but remains anisotropic for the finite chain sizes investigated.
Journal of Organometallic Chemistry, 2007
arXiv (Cornell University), May 25, 2024
We identify for visco-elasto-plastic (VEP) glassy polymers, physical phenomena during Berkovich n... more We identify for visco-elasto-plastic (VEP) glassy polymers, physical phenomena during Berkovich nanoindentation, a locally imposed deformation. Live visuals via in situ nanoindentation indicate mainly sink-in during loading, with pileup after unloading. Scanning Probe Microscopy (SPM) indicates significant volume conserving upflow below the tip, for these high , compliant materials, with compliance correlated high geometric fractional contact, m~0
Bulletin of the American Physical Society, 2016
densely crosslinked networks, macroscopic yield is a transition from deformations of bond lengths... more densely crosslinked networks, macroscopic yield is a transition from deformations of bond lengths and angles, to cooperative deformation of multiple effective network chains via bond torsions. In this work, we examine this yield in terms of the "activation number", ν, of microscopic effective chains between crosslinks. ν is the number of effective network chains, in one Eyring activation volume, V*. It is thus a measure of the number of network chains activated at yield, for cooperative deformation. Microcompression experiments have been performed on SU-8 micropillars, to determine its V* value. SU-8 is an important epoxy thermoset, which is used extensively in the microelectronics industry, in microfluidics and microelectromechanical systems (MEMS). The effective chain length based on Arruda and Boyce's 8-chain model, compares well with the rms length, obtained by chain conformer analyses. We find that ν ∼ 2-4, at room temperature, for DGEBA-based epoxies including SU-8 and DGEBA-amine networks, over a range of network junction functionalities and V*. That ν corresponds very well with the reduced temperature, T/Tg, also demonstrates its viability as a molecular descriptor of yield in densely crosslinked thermosets.
Polymer, 2016
In densely crosslinked glassy thermosets, at yield, bond length and bond angle deformations, tran... more In densely crosslinked glassy thermosets, at yield, bond length and bond angle deformations, transition to cooperative deformation of multiple effective network chains via bond torsions. We introduce here the activation number, n, the ratio of the Eyring activation volume, V*, to the network chain volume, V c , as a measure of the number of network chains, 'activated' at yield for cooperative deformation. We find that n~2e4, at room temperature, for DGEBA-based epoxies including SU-8 and DGEBA-amine networks, over a range of network junction functionalities and V*. This first reported analysis of yield at various strain rates, of the octafunctional epoxy, SU-8, is via our compression experiments on fabricated micropillars. Molecular validation of our bulk density based analysis to determine V c , is achieved by mapping to the eight-chain network model of Arruda and Boyce. We thus identify the activation number as a universal molecular descriptor of yield in densely crosslinked glassy thermosets.
Macromolecules, 2006
We present dynamic Monte Carlo lattice simulations of the coil to globule collapse of single chai... more We present dynamic Monte Carlo lattice simulations of the coil to globule collapse of single chains of a copolymer comprising monomer units, m and c, wherein there is a net attractive interaction between c-units. As the copolymer is cooled, the solvent quality becomes poorer, and the size of the chain decreases, driven by the net mm and cc attractions. The strong cc attraction increases the overall solvophobicity of the chain relative to a homopolymer and, therefore, copolymers collapse more abruptly and at a higher effective temperature relative to homopolymers. We compare copolymers with homopolymers by rescaling collapse data to the same θ values to account for the effect of overall solvophobicity. This comparison shows that the behavior of copolymers and the corresponding homopolymers is identical as the chain size reduces from high temperatures to the θ value. Beyond θ, copolymers with c-content < ∼50% collapse more abruptly than either homopolymer, after accounting for the difference in overall solvophobicity. Collapse of copolymers containing higher c-content is dominated entirely by the cc attractions, and these chains behave qualitatively like homopolymers with a higher effective solvophobicity. Analysis of the chain structure during collapse provides a structural reason for the qualitative change in copolymer collapse at low c-content. When such copolymers are cooled below θ, the c-units rapidly aggregate to form an isotropic, compact core surrounded by an anisotropic solvated shell of m-units. The shell densifies as the copolymer is further cooled, but remains anisotropic for the finite chain sizes investigated.
The Journal of Chemical Physics, 2009
The effect of “sticky” additives (viz., those that have attractive interactions with the polymer)... more The effect of “sticky” additives (viz., those that have attractive interactions with the polymer) on polymer crystallization, has been investigated by dynamic Monte Carlo (DMC) simulations. Additive-polymer attractive interactions result in a slowing down of the polymer chain diffusivity in the melt state. Our results show that with increasing additive stickiness, polymer crystallinity decreases monotonically, and thinner crystallites form, viz., crystallization is inhibited by the presence of sticky additives. Unusually, the observed “specific heat” peak at the phase transition shows nonmonotonic behavior with additive stickiness, and exhibits a maximum for intermediate values of additive stickiness. While the origins of this unexpected behavior are not clear, we show that it correlates with a large interchange between crystalline and amorphous states of the monomers, in the vicinity of the additives. At this intermediate additive stickiness, we also find that crystallization follo...
The Journal of Chemical Physics, 2007
Monte Carlo simulations show that copolymers with uniformly (or periodically) distributed sticky ... more Monte Carlo simulations show that copolymers with uniformly (or periodically) distributed sticky comonomers collapse “cooperatively,” abruptly forming a compact intermediate comprising a monomer shell surrounding a core of the aggregated comonomers. In comparison, random copolymers collapse through a relatively less-compact intermediate comprising a comonomer core surrounded by a fluffy monomer shell that densifies over a wide temperature range. This difference between the collapse pathways for random and uniform copolymers persists to higher chain lengths, where uniform copolymers tend to form multiple comonomer cores. In this paper, we describe the formation of such an intermediate state, and the subsequent collapse, by recognizing that these arise from the expected balance between comonomer aggregation enthalpy and loop formation entropy dictated by the chain microstructure.
A number of people have contributed to this work and helped me see it through completion. I'm in ... more A number of people have contributed to this work and helped me see it through completion. I'm in their debt. I'm especially grateful to my advisor, Dr. Sue Ann Bidstrup who gave me the opportunity to pursue research in this area. Dr. Bidstrup also provided the guidance and support without which this thesis would not have been possible. She devoted extra time and energy and effort to ensure that in spite of myself, this thesis reflected my best effort. For this too I'm grateful. I would like to extend my sincere appreciation to the Dr. A. S. Abhiraman and Dr. Paul Kohl for serving on my Reading Committee. Their advice and suggestions were very helpful in developing this work. Sincere thanks to Dr. Abhiraman for the moral support he gave me and his concern. Materials donation from E. 1. Du Pont de Nemours and Company (Inc.), Electronics Division and financial support from Manufacturing Research Center at Georgia Tech and the Presidential Young Investigator Program of the National Science Foundation is gratefully acknowledged. Special thanks are extended to my group-mates and friends-Jim Companik, Linda Lin and Joycelyn Simpson. Your contributions from the brainstorming sessions, personal moral support and cooperation have been invaluable. I'm especially indebted to my officemate Linda Lin. She has always been there (being available at ungodly hours to prepare the samples for FTIR spectroscopy being one of the many cases in point). Her friendship has been a privilege. Special thanks also to Linda Lin and office-mate and friend Tom Hodge for assistance in proof reading this thesis. I would like to take this opporturlity to thank Steve Hardaker for training me to use the FTIR Spectrometer and helping me learn to interpret spectroscopy data. Life in the USA has been a wonderful experience. I'll always treasure the many friendships I've made in my two and a half year stay. Like I always tell the new Indian students before they find out for themselvesthe natives are a very friendly bunch. Numerous special friendships have developed within the Indian community also during the course of this phase of my pursuit of graduate study and have made my stay here quite enjoyable: Rangaswamy Rajamanickam (Ranga), Nilesh Shroff, Pankaj Upadhyay, Ram Kumar Ramesh (Ravan), Vishnu Vandana, Anuradha Khawas (Anu), Bodhisattwa Mukherjee (Bodhi), Arun Mangalam (Sacold friend and new too), the Dalal family (Satishbhai, Smitaben, Tanvi and Nija). I extend my sincere gratitude to my parents, Jayantkumar and Uma Nanavati, my brother, Kaushik Nanavati, my late uncle, Vasantlal Nanavati and my grandmothers, Navnitben Nanavati and Nirmala Khandwala for their continued love and moral support from across the seven seas. You are all priceless jewels in the treasury of my life.
Journal of Computational Chemistry, Sep 1, 2007
Database-assisted ab initio protein structure prediction methods have exhibited considerable prom... more Database-assisted ab initio protein structure prediction methods have exhibited considerable promise in the recent past, with several implementations being successful in community-wide experiments (CASP). We have employed combinatorial optimization techniques toward solving the protein structure prediction problem. A Monte Carlo minimization algorithm has been employed on a constrained search space to identify minimum energy configurations. The search space is constrained by using radius of gyration cutoffs, the loop backbone dihedral probability distributions, and various secondary structure packing conformations. Simulations have been carried out on several sequences and 1000 conformations have been initially generated. Of these, 50 best candidates have then been selected as probable conformations. The search for the optimum has been simplified by incorporating various geometrical constraints on secondary structural elements using distance restraint potential functions. The advantages of the reported methodology are its simplicity, and modifiability to include other geometric and probabilistic restraints.
In densely crosslinked glassy thermosets, at yield, bond length and bond angle deformations, tran... more In densely crosslinked glassy thermosets, at yield, bond length and bond angle deformations, transition
to cooperative deformation of multiple effective network chains via bond torsions. We introduce here the
activation number, n, the ratio of the Eyring activation volume, V*, to the network chain volume, Vc, as a
measure of the number of network chains, ‘activated’ at yield for cooperative deformation. We find that
n ~ 2-4, at room temperature, for DGEBA-based epoxies including SU-8 and DGEBA-amine networks,
over a range of network junction functionalities and V*. This first reported analysis of yield at various
strain rates, of the octafunctional epoxy, SU-8, is via our compression experiments on fabricated
micropillars. Molecular validation of our bulk density based analysis to determine Vc, is achieved by
mapping to the eight-chain network model of Arruda and Boyce. We thus identify the activation number
as a universal molecular descriptor of yield in densely crosslinked glassy thermosets.
We present dynamic Monte Carlo simulations of the collapse of copolymers containing sticky comono... more We present dynamic Monte Carlo simulations of the collapse of copolymers containing sticky comonomers, c. There is a qualitative difference in the transition depending on c content. For c content >˜ 50%, copolymer collapse is qualitatively similar to that observed for homopolymers, when rescaled to account for comonomer solvophobicity. However, collapse of copolymers with c < ˜50% is qualitatively steeper
The effect of additive particles on polymer crystallization has been investigated using lattice d... more The effect of additive particles on polymer crystallization has been investigated using lattice dynamic Monte Carlo simulation. Additives are compatible with the polymer matrix (viz. there is an attractive ``sticky'' interaction between additives and monomers) and, additive particles have the same size as a monomer. Polymer crystallization is strongly influenced by both additive fraction, x and the additive-monomer interaction strength,
Polyhedron, 2007
Phenoxy-imines are a versatile class of ligands that display a truly impressive range of diverse ... more Phenoxy-imines are a versatile class of ligands that display a truly impressive range of diverse applications spanning from bioinorganic chemistry [1] to coordination chemistry [2], chemical catalysis [3], [4], [5], [6] and [7], and materials related applications [8] and [9]. Equally ...
Polymer Composites, 2011
Solid state polymerization (SSP) is an established procedure to increase the molecular weight (MW... more Solid state polymerization (SSP) is an established procedure to increase the molecular weight (MW) of polyesters [1] (eg, poly (ethylene terephthalate) (PET) [2], poly (butylene terphthalate) [3]), polycarbonates [4], polyamides [5] such as nylon [6, 7], etc. During SSP, ...
The freely orienting model of a polymer chain is generalized by considering the distribution of v... more The freely orienting model of a polymer chain is generalized by considering the distribution of vector lengths and polarizabilities of the statistical segments in the chain. These distributions for a statistical segment have been computed for polyethylene and poly(cis-isoprene) by using Flory's RIS (Rotational Isomeric States) approximation and Denbigh's bond polarizabilities. Two methods, based on distributions of finite chains of freely orienting constant length segments, are used to compute the chain length distribution from the length distribution of statistical segments, (i) an exact method, adopting Chandrasekhar's approach of Fourier transforms of segment vector distributions, and (ii) an alternative approach, based on the generalization of James' and Guth's expression for work of chain deformation. Using the computationally more viable latter approximation, the moments of the chain vector length distributions and chain polarizability are determined f...
Polymer Engineering & Science, 2011
Polymer Engineering & Science, 2011
Polymer Chemistry, 2010
We report here our investigations of the mechanism and kinetics of N-heterocyclic molecules as in... more We report here our investigations of the mechanism and kinetics of N-heterocyclic molecules as initiators for PLA synthesis. This is a step towards employing life saving drugs based on N-heterocyclic functional groups, to generate carrier-drug complexes in situ, via ring ...
Macromolecules, 2006
We present dynamic Monte Carlo lattice simulations of the coil to globule collapse of single chai... more We present dynamic Monte Carlo lattice simulations of the coil to globule collapse of single chains of a copolymer comprising monomer units, m and c, wherein there is a net attractive interaction between c-units. As the copolymer is cooled, the solvent quality becomes poorer, and the size of the chain decreases, driven by the net m-m and c-c attractions. The strong c-c attraction increases the overall solvophobicity of the chain relative to a homopolymer and, therefore, copolymers collapse more abruptly and at a higher effective temperature relative to homopolymers. We compare copolymers with homopolymers by rescaling collapse data to the same θ values to account for the effect of overall solvophobicity. This comparison shows that the behavior of copolymers and the corresponding homopolymers is identical as the chain size reduces from high temperatures to the θ value. Beyond θ, copolymers with c-content < ∼50% collapse more abruptly than either homopolymer, after accounting for the difference in overall solvophobicity. Collapse of copolymers containing higher c-content is dominated entirely by the c-c attractions, and these chains behave qualitatively like homopolymers with a higher effective solvophobicity. Analysis of the chain structure during collapse provides a structural reason for the qualitative change in copolymer collapse at low c-content. When such copolymers are cooled below θ, the c-units rapidly aggregate to form an isotropic, compact core surrounded by an anisotropic solvated shell of m-units. The shell densifies as the copolymer is further cooled, but remains anisotropic for the finite chain sizes investigated.
Journal of Organometallic Chemistry, 2007