Polyimide semi-crystalline

PET and PEN films have an inherent advantage over the amorphous polymers films being both semi-crystalline and biaxially oriented and typically absorbing approximately 1400 ppm moisture at equilibrium (the exact figure depending on temperature and relative humidity). Polyethersulfone and polyimide films are particularly problematic and will absorb over 1% moisture at equilibrium. 

The high melt viscosity of Larc-TPI at 350°C (10 Pas) prohibits its use as an adhesive for bonding metallic or laminate skins on core honeycombs to make high strength sandwich structures. The work of Chow et al. discussed in Section 4.3.2.2 demonstrates that the melt viscosity of the poly(isoimide) form 21 of Fare TPI is of the order of 10 Pas at 240-250°C, compared to 2 x 10 Pas for polyimide 22 [25]. A semi-crystalline form of Larc-TPI has been obtained by chemical imidisation of the polyamic acid with acetic anhydride and triethylamine [63]. The differential scanning calorimetric curve exhibits an endotherm at 274°C due to melting of the crystalline sites. The value of the initial inherent viscosity (0.22 dl g ) indicates, however, that the lower melt viscosity also results from lower molecular weight polymer. 

Figure 51 shows the CR spectra obtained over the 200-470°C range under the compressive stress of 20 MPa for amorphous and semi-crystalline PMDA-ODA polyimide samples differing by their preparation technique. Figure 52 shows the CR spectra for some of PI samples in the temperature region of sub-Fg relaxations (20-300°C), including non-cooperative P-relaxation and low-cooperative intermediate segmental relaxations (at temperatures TJ where Tp[Pg.156]

The piezoelectricity in amorphous polymers differs from that in semi-crystalline polymers and inorganic crystals in that the polarization is not in a state of thermal equilibrium, but rather a quasi-stable state due to the ffeezing-in of molecular dipoles. As mentioned by Broadhurst and Davis (55), four criteria are essential to make an amorphous polymer exhibit piezoelectric behavior. First, molecular dipoles must be present. As seen in Table 1, these dipoles are typically pendant to the polymer backbone as are the nitrile groups in PAN, PVDCN-VAC, and (p-CN) APB/ODPA. However, the dipoles may also reside within the main chain of the polymer such as the anhydride units in the (P-CN) APB/ODPA polyimide. In addition to a dipole moment X, the dipole concentration N (number of dipoles per unit volume) is also important in determining the ultimate polarization, P , of a polymer. 

Amorphous Semi-crystalline (14%) Polyimide, depending on structure... 

Table A.31 Chemical resistance of polyimide (Pi), polyamide imide (PAI), polyphenylene ether (PPE). and polyether ether ketone (PEEK) sc = semi-crystalline [1039]...

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