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Croll Group Research


Block Copolymer Physics

Block copolymers are made whenever polymers of two or more different varieties are joined together.  The simplest type of block copolymer is a diblock copolymer, made whenever two different polymers are covalently bound at one end.  Their behavior as a material is rich and full of fascinating physics.  For example, when cooled below a phase transition point diblocks spontaneously form nanoscopically ordered patterns, very similarly to certain surfactant or lipid systems.  The physics behind the structure has to do with the interplay between the chemical differences of the two blocks (think of oil and vinegar) and entropy loss (both because of localizing the junction point of the diblock to an interface, and because the polymer chains themselves get squeezed in the process).  While our physical understanding of diblocks has grown significantly in recent years, there remain many interesting and unsolved fundamental problems (fluctuations, confinement, non-equilibrium states ...).  Diblock copolymers are also hoped to be useful as lithographic masks of smaller dimension than can be reached by traditional means.  Because of this technological drive there are also many interesting applied problems in this field.

Pattern Formation and Control

Patterns are everywhere in Nature, appearing on large lengthscales and small lengthscales, on the ground and in the sky and most certainly appear in biology.  What is a pattern though?  Have you ever swirled a glass of wine and noticed that small equally spaced droplets of fluid form on the inside of the glass?  Perhaps you have squeezed some skin together between your fingers and noticed equally spaced wrinkles appear.  Or maybe you have looked at a thin layer of oil heating in a pan and noticed equally spaced 'cells' appear.  These are all examples of simple patterns, which we might formally define as a new lengthscale appearing in a system (i.e. equally spaced 'things').  Ultimately, patterns appear because there are two different energies in a system each of which prefers a different lengthscale.  Since both energies cannot be simultaneously minimized, the system must find some lengthscale between the two intrinsic sizes.  For example, as you squeeze skin together between your fingers the top layer wants to bend - like a sheet of paper does.  The energy that occurs when an object is bent is minimized by the smallest possible curvature (or the biggest length) available.  Skin, however, is connected to the soft tissue below it, which must be stretched as the skin is bent.  Intuitively, stretching is minimized by the shortest possible distance (think of pulling on a spring).  Because the squeezed skin cannot have a large bend and a small stretch at once, it forms wrinkles of an intermediate size.  Finding and characterizing new patterns is both fundamentally exciting, and technologically useful - provided you can control the pattern.  Control is easy though if you understand the physics of the pattern.


Thin Films, Adhesion and Mechanics

Strange things can happen simply because a material is made small.  The most remarkable features of a thin polymer film include a glass transition that is vastly different than in a bulk sample.  The reason for these changes remain a significant challenge for polymer physics and its understanding has broad ranging implications.  An interesting way of phrasing the problem is that measurement of small things is quite difficult.  For example, there are many simple ways of measuring the modulus of a large piece of polystyrene (e.g. stretch it).  However, the same measurement on a nanometer thin sheet is incredibly difficult (and therefore prone to error) because thin sheets are sticky, fragile and dominated mechanically by the surfaces that they contact.  Each of these points represents important areas of current research.  Adhesion, a vast area of research, seamlessly connects gecko's to surface science in a truly interdisciplinary field.  Understanding the stickyness of two surfaces requires a deep understanding of the mechanics of both the adhering object and the apparatus (i.e. the gecko) used to test the adhesive forces.  In other words, study of adhesion provides a rational framework to attempt to answer fundamental questions of 'thinness'.

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