Influence of the molecular weight on the thermal and mechanical properties of ethylene/norbornene copolymers

Effect of the polar group content on the glass transition temperature of ROMP copolymers

Polar groups have long been recognized to greatly influence the glass transition temperature (T_g) of polymers, but understanding the underlying physical mechanism remains a challenge. Here, we study the glass formation of ring-opening metathesis polymerization (ROMP) copolymers containing polar groups by employing all-atom molecular dynamics simulations. We show that although the number of hydrogen bonds (N_HB) and the cohesive energy density increase linearly as the content of polar groups (f_pol) increases, the T_g of ROMP copolymers increases with the increase of f_pol in a nonlinear fashion, and tends to plateau for sufficiently high f_pol. Importantly, we find that the increase rate of Gibbs free energy for HB breaking gradually slows down with the increase of f_pol, indicating that the HB is gradually stabilized. Therefore, T_g is jointly determined by N_HB and the strength of HBs in the system, while the latter dominates. Although N_HB increases linearly with increasing f_pol, the HB strength increases slowly with increasing f_pol, which leads to a decreasing rate of increase in T_g.

Nanowear on polymer films of different architecture

In this paper, we describe atomic force microscope (AFM) friction experiments on different polymers. The aim was to analyze the influence of the physical architecture of the polymer on the degree and mode of wear and on the wear mode. Experiments were carried out with (1) linear polystyrene (PS) and cycloolefinic copolymers of ethylene and norbornene, which are stabilized by entanglements, (2) mechanically stretched PS, (3) polyisoprene-b-polystyrene diblock copolymers, with varying composition, (4) brush polymers consisting of a poly(methyl methacrylate) (PMMA) backbone and PS side chains, (5) PMMA and PS brushes grafted from a silicon wafer, (6) plasma-polymerized PS, and (7) chemically cross-linked polycarbonate. For linear polymers, wear depends critically on the orientation of the chains with respect to the scan direction. With increasing cross-link density, wear was reduced and ripple formation was suppressed. The cross-linking density was the dominating material parameter characterizing wear.

Thermodynamics and rheology of cycloolefin copolymers

Cycloolefin copolymers of ethylene and norbornene, with norbornene content in the range from 36 to 62 mol %, were studied with respect to the thermal, thermodynamic, and rheological properties using differential scanning calorimetry, pressure-volume-temperature, and dynamic mechanical measurements. All copolymers obey the principle of time-temperature superposition, i.e., they can be considered as thermorheologically simple except for a temperature range in the vicinity of T(g). Despite this, the results on (i) the ratio of activation energies E(V)(*)/H(*) used to quantify the origin of the liquid-to-glass transition, (ii) the pressure coefficient of the glass temperature T(g)(P), and (iii) the dynamic fragility m suggest increasing dynamic heterogeneity with increasing norbornene content that is driven by the structural heterogeneity along the backbone.