Crystalline Characteristics and Their Influence in the Mechanical Performance in Poly(ε-Caprolactone) / High Density Polyethylene Blends.

Blends of poly(ε-caprolactone) (PCL) and high-density polyethylene (HDPE) have been prepared at different compositions in order to assess the effect of HDPE on gas transport and mechanical behaviors of PCL. Previous to this evaluation, a complete morphological, structural, and thermal characterization were performed using techniques, including SEM, contact angle, FTIR, differential scanning calorimetry, and X-ray diffraction with synchrotron radiation at small and wide angles. Low HDPE incorporations allow interactions to be established at interfaces in the amorphous regions and the enhancement of the mechanical performance. Consequently, the addition of a small amount of HDPE (ranging from 5 to 10 wt%) appears to be appropriate in certain bio-applications where a higher mechanical behavior is required.

Extraordinary mechanical performance in disentangled UHMWPE films processed by compression molding.

An approach to obtain disentangled ultra-high molecular weight polyethylene (UHMWPE) films is proposed using a common compression molding. For that, disentangled UHMWPE nascent powders from reactor are processed at temperatures lower than the main melting peak and at high pressure. Then, disentangled UHMWPE films obtained from homogeneous polymerization powders and from those that incorporate SBA-15 mesoporous silica can be easily achieved by this simple methodology. These disentangled UHMWPE based materials show very high crystallinity and, consequently, outstanding elastic modulus and hardness, both further increasing by presence of mesoporous SBA-15 in the hybrids.

Mechanical and Transport Properties of Poly(propylene-co-1-heptene) Copolymers and Their Dependence on Monoclinic and/or Mesomorphic Polymorphs.

Poly(propylene-co-1-heptene) copolymers have been evaluated in a wide 1-heptene content range. Two different polymorphs can be observed in this interval under the processing conditions imposed: monoclinic crystallites and mesomorphic entities. The unique presence of one or the other lattice at specific content or the coexistence of both of them at the intermediate composition interval is a key factor affecting some of the exhibited properties. The influence is observed in characteristics mainly dependent on either crystalline structure, like rigidity, or the amorphous regions, as location and intensity of glass transition. To get a deeper understanding, the transition from mesophase to monoclinic crystallites has also been studied by DSC and by WAXS and SAXS experiments using synchrotron radiation. It is observed that this transformation is only partial and just involves one-third of the initial ordered mesomorphic entities. Permeability to different gases is mainly dependent on amorphous content and hindrance that monoclinic or mesomorphic entities impose on those disordered regions.

Relaxation response of polymers containing highly flexible side groups monitored by broadband dielectric spectroscopy.

The relaxation behavior of poly(5-acryloxymethyl-5-methyl-1,3-dioxacyclohexane), a polymer containing highly flexible side groups, is studied by broadband dielectric spectroscopy in the frequency and temperature ranges 10(-1)-10(9) Hz and 123-473 K, respectively. Above the glass transition temperature T(g) the dielectric loss in the frequency domain exhibits a prominent alpha absorption, followed in increasing order of frequencies by two secondary absorptions called beta and gamma. At temperatures slightly higher than T(g), the a relaxation is well separated from the beta, but as temperature increases overlapping between both relaxations augments forming an alphabeta absorption in the vicinity of 420 K. This latter absorption displays a shoulder on its high-frequency side corresponding to the y relaxation. The strength of the a relaxation decreases with increasing temperature, eventually vanishing at the temperature at which the alphabeta absorption is formed. The time retardation spectra of the isotherms are calculated and further used to facilitate the deconvolution of the overlapping relaxations. The fact that the temperature dependence of the beta relaxation also describes that of the alphabeta absorption suggests that both relaxations have the same nature. It seems that as temperature increases, the a relaxation feeds on the beta absorption until its complete disappearance. The gamma relaxation, in turn, seems to increase at the expense of the alphabeta process at high temperature.