What is a Polymer? – Basic Nature Of Polymers

 

What is a Polymer? – Basic Nature Of Polymers

 

 

CHEMICAL COMPOSITION

The  term  polymer  denotes  a  molecule  that  is made  up of  many(poly) parts(mers). The  mer ending represents the simplest repeating chemical  structural  unit  from  which  the  polymer  is composed. Thus poly(methy1 methacrylate) is  a  polymer having chemical structural units derived from  methyl  methacrylate, as  indicated  by  the simplified reaction and structural formula I.

What is a Polymer

The  molecules  from  which  the  polymer  is constructed  are  called  monomers  (one  part). Polymer molecules may be prepared from a mixture  of  different  types  of  monomers.  They  are called  copolymers if  they  contain  two  or  more different chemical units  and  telpolymers if  they contain three  different units, as indicated by the structural formulas 11  and 111.

What is a Polymer




As  a convenience  in expressing the structural formulas of  polymers, the mer units are enclosed in  brackets,  and subscripts such as  n,  m,  and p represent the average number of the various mer units  that make  up the polymer molecules. Notice  that  in  normal  polymers  the  mer  units  are spaced in  a random  orientation  along the polymer  chain.  It is possible, however,  to  produce copolymers  with  mer  units  arranged  so  that  a large number of  one mer type are connected to a large  number  of  another mer  type. This  special type of  polymer is called a blockpolymer. It also is possible to produce polymers having mer units with a special spatial arrangement with respect to the adjacent units; these  are called stereospecific polymers.

MOLECULAR WEIGHT

The molecular weight of  the polymer molecule, which equals the molecular weight of  the various mers multiplied by the number of  the mers, may range  from  thousands  to  millions  of  molecular weight units, depending on the preparation conditions. The higher the molecular weight  of  the polymer made from a single monomer, the higher the degree of  polymerization. The term polymerization is often used in a qualitative sense, but the degree  of  polymerization  is  defined  as  the  total number  of  mers in a polymer  molecule. In general,  the  molecular  weight  of  a  polymer  is  reported as the average molecular weight because the number of  repeating units  may  vary  greatly from  one molecule to another. As  would be  expected, the fraction  of  low-, medium-, and high- molecular-weight molecules  in  a  material  or, in other words, the  molecular  weight  distribution, has  as  pronounced  an  effect  on  the  physical properties as the average molecular weight does.

Therefore  two  poly(methy1  methacrylate)  samples can have the same chemical composition but greatly different physical properties because one of  the  samples  has  a  high  percentage  of  low- molecular-weight molecules,  whereas  the other has a high  percentage of  high-molecular  weight molecules. Variation in the molecular weight distribution  may  be  obtained  by  altering  the  polymerization  procedure. These materials therefore do  not  possess  any  precise  physical  constants, such  as  melting  point,  as  ordinary  small  molecules do. For example, the higher the molecular weight,  the  higher  the  softening  and  melting points  and the stiffer the plastic.




SPATIAL  STRUCTURE

What is a Polymer

In  addition to chemical  composition and molecular  weight, the  physical  or  spatial  structure  of the  polymer  molecules  is  also  important  in  determining  the  properties  of  the  polymer. There are  three  basic  types  of  structures:  linear, branched,  and cross-linked. They are illustrated in Figure as segments  of  linear, branched,  and cross-linked polymers. The linear homopolymer has mer units of  the same type, and the random copolymer  of  the  linear  type  has  the  two  mer units randomly  distributed  along the chain. The linear block  copolymer has segments, or blocks, along the chain where the mer units are the same. The  branched  homopolymer  again  consists  of the  same mer units, whereas the graft-branched copolymer consists of one type of mer unit on the main  chain  and  another  mer  for  the  branches. The  cross-linked  polymer  shown  is  made  up of  a  homopolymer  cross-linked  with  a  single crosslinking  agent.




The linear  and branched  molecules are separate and discrete, whereas the cross-linked  molecules are a network structure that may result in the polymer’s becoming one giant molecule. The spatial  structure  of  polymers  affects  their  flow properties,  but  generalizations  are  difficult  to make  because  either  the  interaction  between linear  polymer  molecules  or  the  length  of  the branches  on  the  branched  molecules  may  be more important  in a particular  example. In  general, however, the cross-linked  polymers flow at higher  temperatures  than  linear  or  branched polymers.  Another  distinguishing  feature  of some  cross-linked  polymers  is  that  they  do not absorb  liquids  as  readily  as  either  the  linear  or branched  materials.




An  additional method  of  classifying polymers other than by  their  spatial structure is  according to  whether  they  are  thermoplastic  or  thermosetting.  The  term  thermoplastic  refers  to  polymers that may be softened by heating and solidify  on  cooling,  the  process  being  repeatable.

Typical  examples  of  polymers  of  this  type are  poly(methy1  methacrylate),  polyethylene- polyvinylacetate, and polystyrene. The term thermosetting  refers  to  plastics  that  solidify  during fabrication but cannot be softened by reheating. These  polymers  generally  become  nonfusible because  of  a  crosslinking  reaction  and the  formation  of  a spacial  structure. Typical dental examples  are  cross-linked  poly(methy1 methacrylate),  silicones, cis-polyisoprene, and bisphenol A-diacrylates. Polymers  as  a  class  have  unique  properties, and  by  varying  the  chemical  composition,  molecular weight, molecular-weight  distribution, or spatial arrangement of the mer units, the physical and mechanical properties  of  polymers  may  be altered.




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Adnan Aftab

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