Crystallization and ordering processes in polyhydrocarbons with chemically irregular chains (original) (raw)
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Crystals and Crystallinity in Polymers
2013
Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the 2 Packing of Macromolecules in Polymer Crystals 88 2.1 General Principles, 88 2.2 The Principle of Density (Entropy)-Driven Phase Formation in Polymers, 92 CONTENTS vi CONTENTS 2.3 Symmetry Breaking, 96 2.4 Impact of Chain Folding on Crystal Structure Symmetry, 103 2.5 Frustrated Polymer Crystal Structures, 107 2.6 Chiral Crystallization of Polymers with Helical Chain Conformations, 110 2.7 Packing Effects on the Conformation of Polymer Chains in Crystals: The Case of Aliphatic Polyamides, 113 References, 118 3 Methods in Crystal Structure Determination from X-Ray Diffraction 123
Temperature and Orientation Induced Polymorphic Behavior of Syndiotactic Polypropylene
Macromolecules, 2005
Syndiotactic polypropylene (sPP) was rapidly quenched from the melt to room temperature, obtaining films in the disordered form I, with the chains in helical conformation. A significant portion of chains in the trans-planar mesophase was found, as evidenced by X-ray diffraction and FTIR spectroscopy. The quenched films were drawn to a draw ratio of 6 at 25, 60, and 90°C and analyzed before and after releasing the tension. A large shrinkage was observed unhooking the fibers, and this parameter, expressed as (lmaxlf)/lmax, almost linearly decreased on increasing the drawing temperature. The crystalline phase composition was investigated by diffraction techniques in both the fixed and relaxed fibers. The fully extended fibers drawn at all the investigated temperatures contain a significant amount of the transplanar mesophase while the fraction of the crystalline trans-planar form III decreases on increasing the drawing temperature and it is absent in the fiber drawn at 90°C. Upon relaxation, we found evidence that the trans-planar mesophase in the fixed fiber crystallizes into form II while form III in the fixed fibers tends to transform into the trans-planar mesophase. An infrared and dynamic-mechanical analysis on the fibers was also performed to confirm the structural results. The analysis evidenced that on increasing the drawing temperature, the mesophase in the fixed fibers more and more transforms into the helical form II. On the other hand trans-planar chains remain in the amorphous fraction of all the relaxed fibers. The experimental results allow one to better clarify the role of the trans-planar mesophase in phase transitions of sPP and, taking into account also literature data, to propose a more detailed scheme of transformations between different polymorphs of this polymer.
Structure of crystalline polymers with unbranched long side chains
1971
The structure and thermodynamic properties of atactic and isotactic acrylic and methacrylic polymers containing 16-18 carbon atoms in the n-aliphatic side chains, and of ccpolymers of hexadecyl acrylate with isopropyl acrylate were studied by means of x-ray and differential thermal analysis. The crystallization of branched acrylic and methacrylic polymers and of acrylic copolymers proceeds in the form of a hexagonal crystal, regardless of the configuration of the backbone chain. Methods of ordering branched macromolecules are proposed, and the melting points, heats and entropies of fusion determined. The role of flexibility of the backbone chains in ordering and the crystallization processes was determined. I n the case of poly(n-alkyl acrylates) the backbone chain is involved in the crystalline lattice; this is not the case in methacrylates and copolymers of hexadecyl acrylate with isopropyl acrylate. Some similarity war assumed between the structure of biopolymers and synt.hetic branched polymers.
1991
Works on strictly uniform ultra-long n-alkanes enabled the exploration of the onset of chain folding with increasing molecular length. It was established that folding sets in beyond a certain chain length, more specifically dependant on crystallization temperature (T,), starting in an initially irregular chain deposition, reorganizing subsequently into a strictly quantized conformation of integral fractional fold lengths through either thickening or thinning of the crystal. In the final reorganized stage the folds are regular and sharp. The isothermal crystallization rates for extended chain crystals were found to go through a maximum followed by a minimum with decreasing T, where chain folding takes over. This remarkable rate inversion, observed for c 2 4 6 H494 and c198 H398, so far, occurs both for solution and melt crystallization and could be verified for both primary nucleation and crystal growth. We interpret it in terms of a "self poisoning" phenomenon, where chain depositions, which occur transiently in the "wrong" conformation, are blocking the nucleation and growth of the crystal, a phenomenon also reflected in the reversal of the temperature dependence of isothermal refolding on crystallization from solution. Rate reversals of all these kinds promise to be basic to our understanding and are giving rise to recent alternative explanations from elsewhere which are being quoted and discussed.
Macromolecular Symposia, 2001
A comparative analysis of the polymorphic behavior of samples of isotactic polypropylene (iPP) prepared with heterogeneous Ziegler-Natta catalysts and with a single-center homogeneous metallocene catalyst is presented. Different samples of Ziegler-Natta iPP, prepared with MgCl2-supported catalysts modified by adding different Lewis bases, have been fractionated by extraction with boiling solvents. The irregular fraction, insoluble in diethyl ether and soluble in hexane, crystallizes from the melt almost totally in the γ form. The more stereoregular fractions crystallize instead basically in the R form. This confirms that, even in the case of Ziegler-Natta iPP samples, the γ form may develop by melt-crystallization at atmospheric pressure in fractions containing a high concentration of defects. The relative amount of γ form crystallized from the melt is, however, much lower that that observed in samples of metallocene-made iPP containing comparable amount of defects. Since the γ form crystallizes in chains having short regular isotactic sequences, these data indicate that in Ziegler-Natta iPP samples the regular isotactic sequences are longer than those present in chains of metallocene-made iPP having a similar overall concentration of defects. The different polymorphic behavior of metallocene and Ziegler-Natta iPP samples is related to the different distribution of defects in the polymeric chains, generated by the different kinds of catalytic systems. While in the metallocene-made iPP the distribution of defects along the chains is random, in Ziegler-Natta iPP samples the majority of the defects are segregated in a small fraction of poorly crystallizable macromolecules or in more irregular portions of the chain, so that much longer fully isotactic sequences can be produced, leading to the crystallization of the R form, even for a relatively high overall concentration of defects. These results confirm the idea that the structural analysis of iPP, in particular the crystallization of the γ form, may give information about the microstructure of the polymer chains. The measure of the maximum amount of γ form crystallized from the melt may be used as an indirect method to evaluate the average length of isotactic sequences. This analysis allows concluding that some fractions of Ziegler-Natta iPP are characterized by chains with a stereoblock microstructure, consisting of regular isotactic sequences linked to more irregular sequences. The latter contain the major part of stereodefects mainly consisting in isolated rr triads, r diads, and longer ...rrrr... syndiotatic sequences. The hypothesis of a stereoblock microstructure for some of these less stereoirregular fractions is also consistent with the high degree of crystallinity observed in the samples crystallized from solution or from the melt, despite the high concentration of defects.
A study of structural order in various polyethylenes at elevated temperatures
1977
Changes in several structural properties of an ethylene homopolymer have been measured and compared with those of ethylene copolymers with simple and complex branchings over a range of temperature from ambient to above the normal 'melting temperature'. In the case of the first two polymeric types, a clear behavioural pattern was observed in which, with rising temperature, a small rotation of the backbone chains was followed by melting and recrystallization, the latter two processes occurring in the range from about 110°C to the melting temperature. In the upper temperature range ethyl branches were also rejected from the crystalline lattice. Around the melting temperature, a change from the orthorhombic to the hexagonal phase was suggested as a possibility. The thermal behaviours of the copolymers with the more complex branchings were found to be more complicated with melting and recrystallization occurring over various temperature ranges. N o simple relationship appeared to exist between the numbers and shapes of the complex branchings and the structural changes produced in the copolymers by increasing temperature.
Possible chain conformations in the crystalline state of a series of mesogenic polymers
Macromolecules, 1987
The chain repetition periods in the crystalline state for the series of polymers [-1,4-C6H4C-(C H~= N-N~(C H~-1 , 4-~6 H 4~~~(~H~n~~~O~]~ with n = 8,9, ..., 13 were determined. Through an analysis of the possible chain symmetries satisfying the equivalence principle and on the basis of simple energetic considerations, chain models of these polymers reproducing the experimental periods are outlined. The resulting conformations are such that the elongation axis of the rigid groups-1,4-CsH4C(CHB)=N-N=C(CHB)-1,4-C6H4is significantly displaced from alignment with the chain axis. A proposal for the quantitative evaluation of the chain "waviness" of the polymers studied is presented.
Die Makromolekulare Chemie Rapid Communications
In the last few years considerable attention has been paid to the experimental study of the liquid-crystalline ordering of polymers containing stiff and flexible fragments in the main chain1s-4). However, until very recently there was no theoretical work on this problem. Only in 1981 a paper by Matheson and Flory appeareds) (see also ref.6)), in which the well-known lattice method7s8) was used to consider the liquidcrystalline transition in a solution of macromolecules containing in the main chain stiff rods, freely-rotating joints between the rods, and flexible spacers. One of the main simplifications used in ref. 5, was the assumption that the flexibility of flexible spacers is independent of the degree of orientational ordering in the solution. This assumption seems to be inconsistent with reality: it is clear that the self-consistent orientational field connected with the liquid-crystalline ordering influences not only stiff fragments of the chain, but also flexible ones. As a result, in the liquid-crystalline phase flexible fragments should become somewhat oriented along the anisotropy axis.
Angewandte Chemie International Edition, 2012
Isotactic polypropylene (iPP) is of enormous and still increasing commercial importance because of its favored combinations of properties, such as good rigidity, high thermal resistance, low density, and ease of processability, which can be achieved at relatively low costs. [1] The wide range of applications of iPP results from its versatility and the variety of possible modifications of the basic material, which already starts in the polymerization reactor [1] and can be further achieved during crystallization and processing. In particular, the full understanding of the crystallization and complex polymorphic behavior made it possible to tailor the properties through control of the crystallization of the different crystalline forms (a, b, and g forms) and of the mesophase.