Viscoelasticity of Polymer Crystals Grown from Dilute Solution

Abstract

Single crystals or crystals like those of various polymers are prepared by slow cooling of dilute solution. The solvents used are as follows; cyclohexanol for polyoxymethylene (POM), ethyl alcohol for polyethylene oxide (PEO), xylene for branched polyethylene (b-PE) and for polypropylene (PP), decalin for polybutene-1 (PB-1), diethylene glycol for polyvinyl alcohol (PVA) and glycerol for nylon 6 (PA-6). The dynamic modulus E' and loss modulus E" are measured at frequencies 3.5, 11, 35, 100 and 138c/s over a temperature range from -180°C to the softening temperature by using the direct-reading dynamic viscoelastometers, Vibron Model DDV-I and II. Measurements have been made of the mat of solution-grown crystals and of the bulk-crystallized or solvent-cast film. The latter has been used for comparison.It has been clarified that remarkable crystalline dispersion appears in all the solution-grown polymer crystals, even for the samples of PEO, b-PE, PB-1 and PA-6, in which the crystalline dispersions are scarcely observed in the bulk-crystallized state. This means that the solution-grown crystal has by far the higher crystallinity, and that there exists the intrinsic relaxation mechanism of the crystal phase for all the crystalline polymers. The crystalline dispersion is usually composed of the low temperature side with a low activation energy and the high temperature side with a high activation energy, as we have already reported on linear polyethylene. Two separate crystalline dispersions are observed for PEO (around 0°C and 50°C) and PA-6 (around 130°C and 200°C).The activation energy for crystalline relaxation ΔH* of various polymers is as follows: 25kcal/mole for PB-1, 30kcal/mole for PE, 48kcal/mole for PP and 58kcal/mole for POM. The empirical relationships between ΔH* and the temperature of the E" maximum at 100c/s, Tc, areΔH*=402 Tc-115000 (cal/mole), and ΔH*=407 Tm-132500 (cal/mole).It is concluded that the structural factors contributing to the increase of Tm or Tc are at the same time to increase the value of ΔH*. ΔH* is related to the activation volume ΔV* by the following equation.ΔH*=(α/β)·ΔV*·Tcwhere α is the volume expansion coefficient of the crystal and β is the compressibility of the crystal. By assuming that the value of α/β for linear PE is generally applicable to the other polymers, ΔV* is evaluated as 520cc/mole for PB-1, 550cc/mole for PE, 820cc/mole for PP and 860cc/mole for POM. These values are converted to the corresponding number of main chain atoms, which are 18, 40, 37 and 87 for PB-1, PE, PP and POM respectively. These values decrease at the low temperature side of Tc and increase at the high temperature side. At the temperature higher than Tc by 10 or 20°C the value of ΔV* almost agrees with that of the molecular segment corresponding to the lamella thickness. In such circumstances the segment is considered to be possible of diffusion within the lamella with resulting change on the lamella in shape like that of an amoeba. This makes possible the initiation of lamella thickening by nucleation mechanism, as presented by Hirai.