1.2 Nanoscale Actors and Their Properties

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reactions in the organic chemistry toolkit in any solvent can be used as a polymeriza-

tion, which would not be possible in the homeostasis-bound cell. On the other end, the

cell’s enzymes are more specific; in a cell, the length of the polymer is absolutely con-

trolled and there will be no side reactions, which is difficult to replicate in an organic

chemistry laboratory.

Synthetic polymers are usually made with just one or a few repeating units (ho-

mopolymer and copolymer, respectively). Homopolymers can have different structures:

linear of course, but also hyperbranched or dendritic (100 % branched) (Figure 1.14).

Copolymers can also be linear, with different repeating units randomly arranged (ran-

dom copolymer), arranged in an alternating fashion (alternating copolymer), or ar-

ranged as blocks of each repeating unit (block copolymer). Alternating copolymers

can also be hyperbranched or dendritic. Block-copolymers can be arranged as graft

copolymers, where a different block is grafted onto a homopolymer backbone.

Generally speaking, the longer the polymer, the higher its Tg or glass transition tem-

perature and the stronger the material. Strength can be further increased by crosslink-

ing the polymer, as already seen with proteins.

As with protein structures, while sections might be crystalline, the rest of it, the ma-

trix, will be amorphous, i. e., an unordered solid with a Tg but no melting point. Linear

polymers will always have a larger Tg than the corresponding branched polymer, and

branched structures will not show a melting point (they don’t crystallize). The toughest

materials are semicrystalline, i. e. contain crystalline regions for strength and an amor-

phous matrix for elasticity. Also, the stronger the intermolecular forces between the

polymers, the stronger the material, i. e., hydrophilic polymers with a lot of hydrogen

bonding are generally stronger than hydrophobic ones that only have van-der-Waals

forces as intermolecular bonds. But hydrophilic polymers generally dissolve in water

unless crosslinked, extremely long, or very crystalline.

A polymer’s solubility depends on its hydrophilicity/hydrophobicity. Depending on

their structure, polymers can also be amphiphilic and/or charged. If the polymer has

conjugated sections, it might be colored (depending on the exact conjugation length).

Conjugated polymers can also be conductors or (more likely) semiconductors. Any twist

in the backbone reduces the conjugation length, therefore the highest conductivity is

found in stiff, crystalline polymers. High crystallinity and molecular weight reduces sol-

ubility, thus making it harder to process these materials. The conductivity of a polymer

can be further increased by adding conjugated structures such as carbon nanotubes.

Structure and function of molecules – carbon compounds. Carbon molecules like car-

bon nanotubes are the other set of structures that are common in nanotechnology (Fig-

ure 1.16). All of these structures are based on a single sheet of conjoined aromatic rings

called graphene. If you stack several sheets together, you form graphite (pencil lead).

When rolled up into a tube, it is a carbon nanotube. Shaped into a continuous hollow

sphere, it is called a “fullerene”, after Buckminster Fuller, who identified the structure.

These structures are special in that they are fully conjugated and thus (semi)conduc-

tive. Graphene has been used for electrodes in nanotechnology for that reason. Also,