Material Properties of Plastics

Journal of Polymer Science Part B: Polymer Physics, 1989

The effect of plastic deformation on the chain dimensions of polymers in the semicrystalline state was investigated using linear hydrogenated polybutadiene (HPB), a model ethylene/butene-1 copolymer having about 40% crystallinity at room temperature. Dilute blends of deuterium-labeled chains with various molecular weights (20,000 5 M 5 214,000) in the same unlabeled matrix ( M = 95,000) were uniaxially stretched at 25°C to extension ratios of a = 2.4 and 4.4. Radius of gyration normal to the stretch direction R I was measured for the labeled chains by small-angle neutron scattering. The molecular extension ratio inferred from these data a , = Ri/R: was significantly smaller than a for short chains ( M < 50,000) but increased to the affine range a , = a for M > 100,000. This variation in a,/a closely parallels the molecular weight dependence of mechanical strength and ductility in HPB over the same range.

Macromolecular Chemistry and Physics, 2003

A simple expression for the composition dependence of the Flory-Huggins interaction parameter of polymer/solvent systems reported earlier is used to model the demixing of polymer solutions into two liquid phases. To this end the system specific parameters ζ and ν of that approach are calculated as a function of temperature using the thermodynamic expressions resulting for the critical conditions on one side and from experime ntally determined critical data for polymers of different molar mass on the other side. By means of data reported for the system cyclohexane/polystyrene it is demonstrated that binodal and spinodal lines are very accurately modeled at low temperatures (UCSTs) and at high temperatures (LCSTs). The parameters obtained from the demixing behavior match well with that calculated from the composition dependence of the vapor pressure at temperatures where the components are completely miscible. Information on the phase separation of the system transdecalin/polystyrene for different molecular weights and at different elevated pressures is used to show that the approach is also apt to model pressure influences. The thus obtained ζ (T;p) and ν (T;p) enable the prediction of the (endothermal) theta temperature of the system as a function of pressure in quantitative agreement with the data directly obtained from light scattering measurements with dilute solutions.

Macromolecules, 1999

Wide-angle X-ray methods have been used to compare the structures of poly(vinyl alcohol) (PVA) with syndiotactic diad (S-diad) contents in the range 51-63%. The fiber diagram of a PVA with 51.2% S-diad content (essentially atactic) is indexed by a monoclinic unit cell with dimensions a) 7.82 (0.03 Å, b) 2.53 (0.01 Å (chain axis), c) 5.52 (0.01 Å, and) 91.5 (0.2°. These dimensions are very similar to those proposed by Bunn (Nature 1948, 161, 929) and Sakurada (Bull. Inst. Chem. Res., Kyoto Univ. 1950, 23, 78). As the S-diad content is increased, the crystallites become larger and less distorted and there is lateral contraction of the crystal structure, suggesting that the chains can pack more efficiently. For a syndiotacticity-rich PVA with a S-diad content of 63.1%, the unit cell dimensions are as follows: a) 7.63 (0.02 Å, b) 2.54 (0.01 Å, c) 5.41 (0.01 Å, and) 91.2 (0.1°. The latter structure has a theoretical density of 1.40 g/mL, which is ∼5% higher than the value of 1.34 g/mL predicted for the atactic polymer. The efficiency of chain packing in the two unit cells has been compared by molecular dynamics modeling of arrays of chain segments of different random sequences, with 50% and 65% S-diad contents. For both structures, the results favor hydrogen bonding similar to that proposed by Bunn rather than that due to Sakurada. NVT calculations show that the larger unit cell is favored for the atactic polymer, while the smaller unit cell is favored for the 65% syndiotactic polymer. A NPT assembly for the atactic model starting in the contracted unit cell expanded and equilibrated close to the observed, lower density structure. Likewise, a starting model for the 65% syndiotactic polymer packed with the dimensions for the atactic polymer was found to adjust to the observed contracted structure. The changes are to be understood in terms of the balance between hydrogen bonds (electrostatic) and van der Waals forces, which vary depending on the tacticity.

Polymer, 2011

In this paper, the polymer chain packing and primitive path (PP) network of uncrosslinked and crosslinked cis-polyisoprene (PI) polymer are analyzed upon employing coarse-grained molecular dynamics simulation. The crosslinking effect is found to enhance intra-chain packing of PI polymers, while weakening their inter-chain packing. Surprisingly, these effects cancel each other in the global packing behavior of this polymeric system. We systematically study the effects of molecular weight (MW) and crosslink density on the PP. Both the PP contour length and number of entanglements per chain, hZi, are found to increase linearly with MW for uncrosslinked cis-PI. The corresponding entanglement molecular length N e of cis-PI is estimated to be 76 AE 1, in good agreement with experimental results. The polymer end-to-end distance, the PP contour length as well as hZi of crosslinked PI are reduced by higher intrachain packing density, compared with uncrosslinked PI, if the crosslinkers are ignored in the PP analysis. At the same time, the tube diameter of crosslinked PI is enlarged by the sparse inter-chain packing. By dividing the crosslinked cis-PI chain network into subchains through crosslinked or crosslinker beads, the PP networks of these partial systems are treated as well. We obtain scaling laws between MW/crosslinking density and hZi for crosslinked PI polymers. The simulation results indicate that the random walk assumption, often encountered during the analysis of PPs, can only be applied to the entanglement-dominated (low crosslink density) polymers. For crosslink-dominated (high crosslink density) polymers, whose subchains have a molecular length below 100, this assumption would imply a greatly overestimated entanglement density; we thus avoid the assumption in our analysis. To our best knowledge, this is the first work to uncover the PP of crosslinked polymers.

Polymer Science U.S.S.R., 1984

Journal of Inorganic and Organometallic Polymers and Materials, 2011

The Monte-Carlo (MC) rotational-isomeric-state (RIS) method developed previously to model the stress-strain behaviour of poly(ethylene terephthalate) (PET) is now applied to short PET chains of between 42 and 84 skeletal bonds. The effects on the radial and probability density distribution functions of the long, flexible virtual bond used to represent the terephthaloyl unit are investigated. The distribution functions generated, based on finite samples of chains, are found to be discontinuous with subsidiary maxima in their tails. The discontinuities lead to uncertainties in the simulated network elasticity properties and, in order to reduce the uncertainty, it is necessary to truncate the distributions at the values of r where they first become discontinuous. The stressstrain properties calculated from the truncated distributions are shown to be consistent with those presented previously for longer PET chains, for which discontinuities in the sampled functions are not apparent.