hemant nanavati | Indian Institute of Technology Bombay (original) (raw)

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Papers by hemant nanavati

Bulletin of the American Physical Society, 2016

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.

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 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 ...

Journal of Organometallic Chemistry, 2007