Chain Structure and Configuration

Chapter 2.  Chain Structure and Configuration

2.9  Analysis of polymers  during mechanical strain 

Far from  the neck region 

In the  neck region 

Many polymers  in the solid (bulk) state undergo strain,  either during processing such as extrusion,  molding, and spinning or when in service  and under load. Studies using solid-state  NMR and FTIR showing how polymers respond  to strain have contributed greatly to  improving their mechanical behavior.

Instrument 

Quantity  Measured 

Reference 

CP MAS ¹³C NMR 

Trans-gauche shifting to higher trans levels with increasing st-polypropylene chain orientation 
 

(a) 
 

CP MAS ¹³C NMR 

Crystallization of poly(tetramethylene oxide) block on orientation of stretched poly(butylene terephthalate)-block-poly(tetramethylene oxide) elastomers (~700% strain) 
 

(b) 
 

FTIR 

Interchain hydrogen bonding in polyurethanes decreasing with increasing strain 
 

(c) 
 

FTIR 

Chain orientation in glassy epoxy resins increasing with plastic deformation (10% strain); absorbances measured parallel and perpendicular to the stretching direction 
 

(d) 

References: (a) P. Sozzani, M. Galimberi, and G. Balbontin, Makromol. Chem., Rapid Commun., 13, 305 (1992). (b) A. Schmidt, W. S. Veeman, V. M. Litvinov, and W. Gabriёlse, Macromolecules, 31, 1652 (1998). (c) S. L. Huang and J. Y. Lai, Eur. Polym. J. 33, 1563 (1997). (d) T. Scherzer, J. Polym. Sci., Part B: Polym. Phys. Ed, 34, 459 (1996). 

Table  2.8 Molecular properties  of polymers during strain

 

Bretzlaff and  Wool (37) performed stress-strain studies on  it-polypropy­lene, finding that the frequency  shift followed the relation 

Av_^CT = v(s) - v(0) = a xS (2.39) 

  where -Av_^s represents the mechanically induced peak frequency shift, a_^x is the mechanically induced frequency shifting coefficient at constant temperature T, and s is the applied uniaxial stress. 

  However, the interpretation of the data was complicated by the existence of anisotropic crystal field forces, in addition to interchain perturbing forces, and the question remains unresolved. 

2.10  Photophysics of polymers 
Photophysics is the science of the absorption, transfer, localization, and emis­sion of electromagnetic energy, with no chemical reactions occurring. By con­trast, photochemistry deals with those processes by which light interacts with matter so as to induce chemical reactions. 
 
The first step, of course, is the absorption of electromagnetic energy, trans­forming it into excited molecular states, 
A + hu = A* 
where A is the molecule to be excited, A* represents the excited state, and hv represents the electromagnetic energy absorbed. 
 
Next most important is energy migration, either along the chain or among the chains. The energy migrates throughout the system of antennas within about 100 ps, being transmitted to the reaction center protein (40). Hence, in polymer photophysics, this phenomenon is termed the "antenna effect." 
 
 

2.10.1  Quenching Phenomena

In situations  where bimolecular encounters dominate, typical  for polymers, such encounters may lead  to an electronic relaxation of the system,  termed quenching. In general, such collisions  may be written

A* +B =  A + B*(2.41)

where the excited  molecule A* encounters another molecule B.  Most often, the bimolecular interaction is  between an excited molecule in the singlet  state and a quencher molecule in the  ground state.

 Chemical reactions involve cross-linking, degradation, and rearrangement. Electronic energy transfer involves exothermic processes, where part of the energy is absorbed as heat, and part is emitted via fluorescence or phospho­rescence from the donor molecule. 
 
 

2.10.2  Excimer Formation

The formation  of an excimer from an excited-state moiety  A* and a ground- state moiety A may  be illustrated as

A* + A  = (AA)*(2.42)

The excimer,  (AA)*, decomposes due to a variety of  interactions, the most important one being  the emission of fluorescence:

(AA)* = 2A  + hvE(2.43)

where the emitted  frequency, v_^E, is lower than the input frequency, the remain­ing energy being required to separate the two moieties and/or heat genera­tion, and h is Planck's constant. 
 
 
 
 
 
 
 

2.10.3  Experimental Studies 

2.10.3.1  Microstructure of polystyrene