Molecular modeling of polymers. IV. Estimation of glass transition temperatures

A method is described which allows molecular modeling to be combined with a group additive property model to estimate glass transition temperatures of linear polymers. T_g is assumed to be a function of conformational entropy and mass moments of the polymer. These two molecular properties are estimated in terms of the torsion angle units composing the polymer using conformational energy calculations. A “universal” T_g equation is derived using 30 structurally diverse polymers and multidimensional linear regression analysis. “Designer” T_g equations are also derived specifically for acrylate and methacrylate polymers. The work described here demonstrates how molecular modeling can be combined with group additivity theory to yield open-ended models that are not restricted by lack of requisite group additive parameters and take advantage of three-dimensional molecular information.     

Interactions between water and polystyrene

Water equilibrium in glassy polystyrene (T_g = 343 K) is described. Water content increases with increasing temperature and increasing water vapour pressure. The heats and entropies of mixing show that the polymer contains both free water and water sorbed on the chains, in small but approximately equal amounts; no noticeable clustering takes place.     

Synthesis and characterization of topologically interesting vinyl polymers

A series of macrocyclic poly(2-vinylpyridine)&s were synthesized by high dilution (∼10⁻⁵ M) coupling of the two-ended living precursor dianions with 1, 2- and 1, 4-bis-(bromomethyl)benzene (1, 2- or 1, 4-DBX). SEC measurements indicate that the macrocycles contain less than 5% linear precursor and that the hydrodynamic size of the macrocycles is substantially (∼30%) less than that of the linear precursor. At very low MW, the sizes of macrocyclic and linear polymers differ less, particularly for the case of the 1, 2-DBX product. Viscometry measurements in THF of linear and macrocyclic polymers also indicated substantial size differences of linear and macrocyclic P2VP. The values of the glass transition temperature (T_g) for both the 1, 2- and 1, 4-DBX macrocycles were found to increase with decreasing MW as qualitatively predicted by entropy calculations. Thus, as the degree of polymerization decreases, the macrocyclic chains become conformationally stiffer and this is reflected by higher T_g values. The differences in T_g between linear and macrocyclic P2VP are quite large (∼40K) around a DP of 40 and decrease to values of a few degrees at a DP of around 200. These differences are diagnostically useful in estimating the linear content of the macrocycles below a DP of 100.     

DDSC Studies on POTMG/PMMA Blends and POTMDM-net-PMMA

Thermal behaviors of POTMDM-net-PMMA and POTMG/PMMA blends were studied by DDSC. T_g of the polymer network was lowered by increasing the POTMDM in feed for copolymerization of POTMDM and MMA. A crystallization peak was observed only when MMA in feed was less than 30%. T_g of POTMG/PMMA was also lowered by decreasing the content of PMMA, however, the change was observed only when PMMA content was more than 70%. These results suggest that thermal transitions of the polymer network are restricted by the mesh size. POTM chains of the polymer network effectively play as a plasticiser. This revised version was published online in August 2006 with corrections to the Cover Date.     

Superselective Adsorption of Multivalent Polymer Chains to a Surface with Receptors

Multivalent polymer chains exhibit excellent prospect in biomedical applications by serving as therapeutic agents. Using three-dimensional (3D) Langevin dynamics simulations, we investigate adsorption behaviors of multivalent polymer chains to a surface with receptors. Multivalent polymer chains display superselective adsorption. Furthermore, the range of density of surface receptors at which a multivalent polymer chain displays a superselective behavior, narrows down for chains with higher density of ligands. Meanwhile, the optimal density of surface receptors where the highest superselectivity is achieved, decreases with increasing the density of ligands. Then, the conformational properties of bound multivalent chains are studied systematically. Interestingly, we find that the equilibrium radius of gyration R_g and its horizontal component have a maximum as a function of the density of surface receptors. The scaling exponents of R_g with the length of chain suggest that with increasing the density of surface receptors., the conformations of a bound multivalent polymer chain first fall in between those of a two-dimensional (2D) and a 3D chain, while it is slightly collapsed subsequently.     

The glass transition temperature of nonstoichiometric epoxy–amine networks

Glass transition temperatures (T_g) of nonstoichiometric epoxy-amine networks based on the diglycidylether of bisphenol A (DGEBA), are analyzed in terms of the network structure. In most cases reasonable predictions of T_g can be made using an empirical equation reported by L. E. Nielsen together with the experimental T_g value of the stoichiometric network and statistical calculations of the concentration of elastic chains. It is stated that in these rigid networks the concentration of elastic chains is the main structural factor associated to the variations of T_g with stoichiometry. For flexible networks based on the diglycidylether of butanediol (DGEBD), the effect of elastic chains on the T_g value is much less significant.     

Internal plasticization of poly(vinyl) chloride using glutamic acid as a branched linker to incorporate four plasticizers per anchor point

Internal plasticization of polyvinyl chloride (PVC) using thermal azide‐alkyne Huisgen dipolar cycloaddition between azidized PVC and electron‐poor acetylenediamides incorporating a branched glutamic acid linker resulted in incorporation of four plasticizing moieties per attachment point on the polymer chain. A systematic study incorporating either alkyl or polyethylene glycol esters provided materials with varying degrees of plasticization, with depressed T_g values ranging from ?1 °C to 62 °C. Three interesting trends were observed. First, T_g values of PVC bearing various internal plasticizers were shown to decrease with increasing chain length of the plasticizing ester. Second, branched internal plasticizers bearing triethylene glycol chains had lower T_g values compared to those with similar length long‐chain alkyl groups. Finally, thermogravimetric analysis of these internally plasticized PVC samples revealed that these branched internal plasticizers bearing alkyl chains are more thermally stable than similarity branched plasticizers bearing triethylene glycol units. These internal tetra‐plasticizers were synthesized and attached to PVC‐azide in three simple synthetic steps. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 1821–1835     

Revisiting the crystallization of poly(2‐alkyl‐2‐oxazoline)s

Poly(2‐alkyl‐2‐oxazoline)s (PAOx) exhibit different crystallization behavior depending on the length of the alkyl side chain. PAOx having methyl, ethyl, or propyl side chains do not show any bulk crystallization. Crystallization in the heating cycle, that is, cold crystallization, is observed for PAOx with butyl and pentyl side chains. For PAOx with longer alkyl side chains crystallization occurs in the cooling cycle. The different crystallization behavior is attributed to the different polymer chain mobility in line with the glass transition temperature (T_g) dependency on alkyl side chain length. The decrease in chain mobility with decreasing alkyl side chain length hinders the relaxation of the polymer backbone to the thermodynamic equilibrium crystalline structure. Double melting behavior is observed for PButOx and PiPropOx which is explained by the melt‐recrystallization mechanism. Isothermal crystallization experiments of PButOx between 60 and 90 °C and PiPropOx between 90 and 150 °C show that PAOx can crystallize in bulk when enough time is given. The decrease of T_g and the corresponding increase in chain mobility at T > T_g with increasing alkyl side chain length can be attributed to an increasing distance between the polymer backbones and thus decreasing average strength of amide dipole interactions. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 721–729     

Interpenetrating blends of rigid aromatic fractal polyamides and nylon-6

Blends of nylon-6 and up to 20% rigid aromatic fractal polyamides (FPs) were prepared by precipitation from a mutual solvent and by two melt-processing procedures. In general, no grafting of the flexible linear nylon chains onto the rigid FPs took place, but in several instances of melt-blending of nylon with FPs whose amine end-groups were exposed, a low level of grafting occurred. The glass transition temperature and the tensile modulus and yield strength of the blends were greatly elevated as function of the FP concentration in the blends. This was demonstrated to be caused by the openness and rigidity of the FPs, and the connectivity of the FP segments through rigid branchpoints. The great porosity of the FPs allows the chains of the amorphous fraction of the nylon to interpenetrate and pass through the FPs, and the stiff segments of the FPs to suppress the chain motions of the nylon, which accounts for the enhanced glass transition temperature (T_g) and tensile properties. When non-porous amorphous silica particles or stiff linear or essentially unbranched zigzag polyamides were blended with the nylon, the T_g of the nylon either did not change at all or changed only very little. Several analytical procedures were used to verify that the nylon chains occupied most of the free space in the pervaded volumes of the FPs in the as-prepared blends and filled this space completely when these blends were compression-molded. The point where the FPs filled all the volume of the amorphous fraction of the nylon-6 was reached between 5 and 7.5% FP concentration. Below this, traces of the original nylon-6 T_g could be occasionally detected. Above it, only the high T_g of the nylon chains interpenetrated in the FPs was detected.     

Birefringence and glass transition temperatures of poly(diethylene glycol terephthalate) networks

A series of polymer networks were prepared by trifunctionally endlinking poly(diethylene glycol terephthalate). The elastomeric properties of these materials were studied at constant temperature using experiments that involve both the elastic force and birefringence. Whereas the stress-strain isotherms show an anomalous increase in the modulus at very high elongation ratios, a downturn appears in the birefringence-strain isotherms at the same extensibilities. These results suggest that the upturn that appears in the force should be attributed to maximum chain extensibility rather than to strain-induced crystallization. A variety of additional thermoelastic experiments were carried out on these networks, to elucidate the dependence of the glass transition temperature on strain. It was found that for the elongation ratios at which the networks exhibit Gaussian behavior, the free-volume effects on the glass transition temperature T_g (decreasing T_g with increasing free volume) offset the conformational effects (increasing T_g with decreasing entropy). However, the contrary occurs in the region where the stress increases anomalously with increasing strain.     

Preparation and properties of silicon containing copolyamides

Copolyamides containing siloxane moieties in main chain were prepared by a melt polycondensation with 1,3-bis(3-aminopropyl)tetramethyldisiloxane (E), hexamethylenediamine (N6), and adipic acid (6). Glass transition temperature (T_g), cold crystallization temperature (T_cc), and melting temperature (T_m) were measured by differential thermal analysis (DTA). The depression of T_m for copolyamide was fitted by the Flory curve. Melting peak remarkably broadens with increasing E6 component in copolyamide. The change of T_g was fitted by the Gibbs and Dimarzio's equation in which the number of flexible bond is considered. The difference between T_g and T_cc increased with increasing E6 component. These DTA studies suggest that the crystallization of N66 component in copolyamide is hindered by the bulky siloxane moiety, while the micro-Brownian motion of amorphous segment is promoted by the flexible siloxane bond. Tensile strength and Young's modulus decreased with increasing E6 component. The solubility in various solvents increased with increasing E6 component. Permeability of oxygen and nitrogen increased with an increase of temperature and E6 component. The separation coefficient of oxygen to nitrogen rapidly increased near 50 mol% of E6 concentration and then leveled out above 70 mol%. The contact angle with water and methylene iodide increased with an introduction of the siloxane moiety into polymer chain.     

Synthesis, characterization and thermoreversible behaviours of poly(dimethyl siloxane)/poly(N-isopropyl acrylamide) semi-interpenetrating networks

Simultaneous and sequential poly(N-isopropyl acrylamide) (PNIPAAm)/poly(dimethyl siloxane) (PDMS) semi-interpenetrating polymer networks (IPNs) with different linear PDMS contents were prepared by free radical polymerization method. Their phase morphologies have been characterized by FTIR, DSC and SEM. The simultaneous semi-IPNs exhibited phase transition temperatures (T_pt) shifted higher temperature from glass transition temperatures (T_g) of their respective homopolymers, suggesting a heterophase morphology and only physical entanglement between the PNIPAAm network and linear PDMS with high molecular weight (Mn≈9000 g/mol). For sequential semi-IPNs, the shift of T_pts towards lower temperature suggested that the chemical interaction between the constituents of the IPNs increased with increasing PDMS content in the network. In addition, these semi-IPNs were characterized for their thermo-sensitive behaviour by equilibrium swelling studies. The results showed that incorporation of hydrophobic PDMS polymer into the thermo- and pH-sensitive PNIPAAm and P(NIPAAm-co-IA) (itaconic acid) hydrogels by semi-IPN formation decreased swelling degrees of IPNs without affecting their LCSTs whereas addition of acrylated PDMS (Tegomer V-Si 2250) as crosslinker instead of N,N^′-methylenebisacrylamide (BIS) into the structures of these hydrogels changed their LCSTs along with their swelling degrees.     

Computer simulation study of the global phase behavior of linear rigid Lennard-Jones chain molecules: comparison with flexible models

The global phase behavior (i.e., vapor-liquid and fluid-solid equilibria) of rigid linear Lennard-Jones (LJ) chain molecules is studied. The phase diagrams for three-center and five-center rigid model molecules are obtained by computer simulation. The segment-segment bond lengths are L = sigma, so that models of tangent monomers are considered in this study. The vapor-liquid equilibrium conditions are obtained using the Gibbs ensemble Monte Carlo method and by performing isobaric-isothermal NPT calculations at zero pressure. The phase envelopes and critical conditions are compared with those of flexible LJ molecules of tangent segments. An increase in the critical temperature of linear rigid chains with respect to their flexible counterparts is observed. In the limit of infinitely long chains the critical temperature of linear rigid LJ chains of tangent segments seems to be higher than that of flexible LJ chains. The solid-fluid equilibrium is obtained by Gibbs-Duhem integration, and by performing NPT simulations at zero pressure. A stabilization of the solid phase, an increase in the triple-point temperature, and a widening of the transition region are observed for linear rigid chains when compared to flexible chains with the same number of segments. The triple-point temperature of linear rigid LJ chains increases dramatically with chain length. The results of this work suggest that the fluid-vapor transition could be metastable with respect to the fluid-solid transition for chains with more than six LJ monomer units.     

Mechanism of generation of photoelastic birefringence in methacrylate polymers for optical devices

We clarified the birefringence properties of poly(methyl methacrylate), poly(ethyl methacrylate), poly(isobutyl methacrylate), poly(cyclohexyl methacrylate), poly(isopropyl methacrylate), and poly(tert‐butyl methacrylate). We demonstrated that the conformational change in polymer molecules that causes orientational birefringence differs from that causing photoelastic birefringence. Orientational birefringence depends mainly on the orientation of the main chains of the methacrylate polymers above T_g. On the other hand, photoelastic birefringence in elastic deformation below T_g depends mainly on the orientation of the side chains while the main chains are scarcely oriented. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2029–2037, 2010     

Equation of state and the thermodynamic properties of liquid polymers: Application of the Hirai-Eyring model

It is shown that the Hirai-Eyring model for the liquid state is capable of accurately describing the p, V, T behavior of liquid polymers in the temperature range over which measurements are now made, and below. Once the parameter choices necessary to accomplish the fit are made for a particular polymer, the excess thermodynamic functions (differences in properties, liquid less solid) are determined by the same parameters. Above the glass transition temperature T_g the volume, excess enthalpy, and square of the excess entropy are predicted by the model to be essentially linear with temperature, in agreement with experiment. Below T_g, these functions do not remain linear (as is usually assumed in extrapolating the equilibrium behavior to low temperatures), but instead they rapidly approach zero in a continuous way as the temperature is lowered. These remarks apply to glass-forming materials composed of small molecules, as well as to polymers. The “paradox” raised by Kauzmann is thus resolved, and the Gibbs-DiMarzio second-order transition appears to be unnecessary.     

Effect of conformational parameters on physical properties of polymers containing pendant phenoxyphthalonitrile substituents

Heteroaromatic polymers are considered to be high performance organic materials due to their unique and highly attractive properties, including outstanding thermal and mechanical resistance, that arise from their aromatic structure and strong interactions between macromolecular chains. Modification or designing new molecular architectures with tailored physico-chemical characteristics allows expanding the applications of these materials in various advanced technologies. Herein, a series of polymers containing bulky phenoxyphthalonitrile pendant units was synthesized and their physical properties were studied and correlated with their conformational parameters, as well as free and van der Waals volumes. For comparison, the related polymers without lateral moieties were also investigated to highlight the effect of bulky substituent on the polymer rigidity. Thus, it is shown that conformational rigidity determines the packing of macromolecules in solid state, and, therefore, the free volume, glass transition, and decomposition temperatures. The values found experimentally for T _g correlate well with those obtained using the conformational rigidity parameters. The dependence of T _g of these polymers on Kuhn segment is described by linear equations, with very good factors of convergence. The correlations established by Monte Carlo method allow obtaining the T _g values for related polymers where the experimental measurement of this parameter is difficult.     

Effect of alkyl side chain length on the properties of polyetherimides from molecular simulation combined with experimental results

Three polyetherimides (PEIs) with the same backbone of Ultem 100 but different lengths of the alkyl side chains were simulated by using molecular dynamics and molecular mechanics techniques to investigate the effect of side chain length on their properties and physical mechanism behind. Simulation results, which are consistent to the experimental data, show that PEI‐5 with four methylene units in each alkyl side chain has higher T_g (glass transition temperature) and higher tensile strength, but lower tensile elongation at break than those of PEI‐6 with five and PEI‐8 with seven methylene units in each alkyl side chain. However, unlike the traditional phenomena, conformational analysis provides that PEI‐5 with the highest T_g gives the highest flexibility to the polymer chain, whereas PEI‐8 with the lowest T_g imparts the lowest flexibility resulting from attachment of longer alkyl side chain increase the rigidity of backbone. From the calculated ratio of the accessible volume to the total volume for each system, the highest ratio of PEI‐8 indicates that long alkyl side chains generate more free volume than short side chains, acting as an internal plasticizer in bulk structure. It is the internal plasticizing effect that is predominantly responsible for the abnormal properties, instead of the rigidity from side chains. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 595–599, 2010     

Influence of macroinitiator’s glass transition temperature on the response times of polymer dispersed liquid crystals

Glass transition temperature (T_g), an important parameter of polymer, was reported to have great influence on the electro-optical properties of polymer dispersed liquid crystals (PDLCs). In this study, macroinitiators with different T_g were synthesised by reversible addition fragmentation chain transfer polymerisation, and used to prepare PDLCs with different T_g block chains. The effect of different T_g of the block chains on response times was investigated. It was found that rise time decreased and decay time increased with the decrease of the block chain’s T_g. We proposed a possible mechanism by which T_g of the block chains influence response times.     

The Meaning of the Glass Temperature of Random Copolymers and Miscible Polymer Blends

The glass transition temperature of random copolymers and miscible polymer blends exhibits generally a non-additive composition dependence, showing both positive and negative deviations of additivity predicted by 'Gordon-Taylor' like equations, among which the Fox relation represents the simplest additivity rule for the glass temperature of these polymeric systems. It is shown that the real T_g vs. composition behaviour of both copolymers and polymer blends can be adapted by a parameterized third order T_g vs. composition equation. The fitting parameter, K₁, of the square concentration term of this equation accounts essentially for the effect of binary hetero-sequences in copolymers and for specific contact hetero-interactions in polymer blends. The fitting parameter of the third order concentration term, K₂, is related exclusively to the effects of heterotriad sequences (copolymers) and conformational entropy changes due to hetero-contact formation (polymer blends), respectively. It is shown that the K₁ parameter correlates roughly with the difference between the solubility parameters of the components. This revised version was published online in August 2006 with corrections to the Cover Date.